New Oxidation Catalysts Comprising Mesoionic Carbenes as Potentially Cooperative Ligands
Final Report Abstract
1,2,3-Triazol-5-ylidenes and isoxazolylidenes comprise new classes of N-heterocyclic carbenes that are promising ligands in catalytically active complexes due to their mesoionic character. This enables these 'abnormal' carbenes to respond to the electronic demand of the metal center and to provide stronger donor properties compared to normal carbenes which opens up various opportunities in catalysis. The triazole and isoxazole ligand precursors were synthesized following a [3+2] cycloaddition protocol starting from alkynes and azides or ethyl nitroacetate, respectively. The different electronics in the heterocycle had a direct impact on functionalization and metallation, indicating a great scope of tenabillty with these types of ligands. For instance the supposedly more basic isoxazolylldene precursors were challenging to be functionalized, for example by halogenation. However, this strategy was successful for the synthesis of triazolylidene complexes. Here the triazole was iodinated, the coordination was then achieved by two different routes, first oxidative addition to a low-valent metal precursor and subsequent methylation or alkylation of the iodotriazole followed by oxidative addition of the iodotrlazolium salt, with this being the first time that triazolylidene complexes were synthesized via oxidative addition. Through this synthesis platinum, palladium and nickel complexes were obtained, indicating the great scope of this metallation procedure. The platinum triazolylidene complex showed promising catalytic activity in the hydration of alkynes, whereas no activity has been observed thus far in hydrosilylation reactions. The alkyne hydration catalysis opens up intriguing synthetic opportunities and should be continued to further investigate the substrate scope of the platinum triazolylidene complex as well as exploiting the vast possibilities for catalytic reactions.
Publications
- Cleavage of typically unreactive bonds with pincer metal complexes, Dalton Transactions 2011. 40, 8733-8744
M. Albrecht, M. M. Lindner
(See online at https://doi.org/10.1039/c1dt10339c) - Oxidative Addition - A Route to Abnonnal 1,2,3-Triazolylidene Complexes of Ni, Pd and Pt, 2012 Symposium on Inorganic Chemistry in Ireland, Galway, Irland
M. M. Lindner, M. Albrecht
